Conventionally, fluid machineries include a scroll compressor used in an air conditioner, as disclosed in Japanese Unexamined Patent Application Publication No. 2005-147101. The scroll compressor includes a fixed scroll and an orbiting scroll including spiral wraps formed on the front surfaces of end plates. In a state that the scroll wraps are in engagement with each other, the orbiting scroll revolves with respect to the fixed scroll without rotating. This revolution compresses the volumes of compression chambers to compress the refrigerant.
On the back surface side of the orbiting scroll of the scroll compressor, a back pressure chamber is formed. The back pressure chamber communicates with a compression chamber in an intermediate pressure state, and the refrigerant at the intermediate pressure is introduced to the back pressure chamber. A predetermined amount of thrust force of the intermediate pressure refrigerant thrusts the orbiting scroll against the fixed scroll to remove a gap between the wraps and the opposed end plates. Further, when the compression chamber becomes at an abnormal high pressure, the abnormal high pressure is released between the wraps and the opposed end plates to the low pressure side.